Nobody has really been able to make a convincing argument whether these sorts of tools haven't lead to large-scale terrorism through bioweapons because the underlying problem is hard (for a sufficiently motivated adversary), or that terrorists don't have the resources/knowledges/skill, and as far as we can tell, the sufficiently motivated adversaries who have tried either failed, succeeded secretly, or were convinced to walk back from the brink due to the potential consequences.

In short there are other ways to negatively affect large numbers of people that are easier, and presumably those avenues are being explored first. But we don't know what we don't know.